[PATCH bpf-next 17/20] libbpf: Let the static linker carry .bpf_cleanup relocations

[Yonghong Song <yonghong.song@linux.dev>]

An object that carries a compiler-emitted exception cleanup table cannot be
linked today. The table's fields are byte offsets into a code section,
materialised by a 32-bit relocation against that section's symbol with the
offset itself as the implicit addend, and the linker rejects both halves of
that: the relocation type is not in the list it accepts, and a relocation
against an STT_SECTION symbol from a non-executable section is an outright
error.

Both spellings of that relocation have to be taken. LLVM emits
R_BPF_64_NODYLD32 for a .long against a section symbol; GNU as emits
R_BPF_64_ABS32, which is what bpf_reloc_type_lookup() maps BFD_RELOC_32 to.
They describe the same value, and the selftests are built with both
compilers.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
 tools/lib/bpf/linker.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/tools/lib/bpf/linker.c b/tools/lib/bpf/linker.c
index 78f92c39290a..e5c06023cb5b 100644
--- a/tools/lib/bpf/linker.c
+++ b/tools/lib/bpf/linker.c
@@ -1036,7 +1036,8 @@ static int linker_sanity_check_elf_relos(struct src_obj *obj, struct src_sec *se
 		size_t sym_type = ELF64_R_TYPE(relo->r_info);
 
 		if (sym_type != R_BPF_64_64 && sym_type != R_BPF_64_32 &&
-		    sym_type != R_BPF_64_ABS64 && sym_type != R_BPF_64_ABS32) {
+		    sym_type != R_BPF_64_ABS64 && sym_type != R_BPF_64_ABS32 &&
+		    sym_type != R_BPF_64_NODYLD32) {
 			pr_warn("ELF relo #%d in section #%zu has unexpected type %zu in %s\n",
 				i, sec->sec_idx, sym_type, obj->filename);
 			return -EINVAL;
@@ -2274,6 +2275,22 @@ static int linker_append_elf_relos(struct bpf_linker *linker, struct src_obj *ob
 						insn->imm += sec->dst_off / sizeof(struct bpf_insn);
 					else
 						insn->imm += sec->dst_off;
+				} else if (sym_type == R_BPF_64_NODYLD32 ||
+					   sym_type == R_BPF_64_ABS32) {
+					__u32 *val;
+
+					/* Two spellings of the one thing: LLVM
+					 * emits NODYLD32 for a .long against a
+					 * section symbol, GNU as emits ABS32
+					 * (bpf_reloc_type_lookup() maps
+					 * BFD_RELOC_32 to it), and the value
+					 * they describe is the same.
+					 */
+					val = dst_linked_sec->raw_data + dst_rel->r_offset;
+					if (linker->swapped_endian)
+						*val = bswap_32(bswap_32(*val) + sec->dst_off);
+					else
+						*val += sec->dst_off;
 				} else {
 					pr_warn("relocation against STT_SECTION in non-exec section is not supported!\n");
 					return -EINVAL;
-- 
2.53.0-Meta